Since the beginning of time man has sought ways to provide a shoe sole that is stable, yet comfortable. Typically, the stability of a shoe sole is enhanced by increasing its rigidity. Comfort is typically enhanced by increasing the cushioning provided in the shoe. Thus, the more stable the shoe the less cushioning that is provided, and conversely, the more cushion that is provided the less stable the shoe. As a result, stability is often sacrificed for the sake of comfort, and vice versa.
In this century, the introduction of shoes designed specifically for athletic purposes has highlighted this problem. Many athletic activities involve running and jumping that translate to high impact forces on the foot. As a result, today's typical athletic shoe sole includes a cushion "midsole" layer that is sandwiched between the insole and outsole layers of the sole. This midsole layer is usually made of a foam material to provide the maximum cushion effect to the foot. Unfortunately, the effect is much the same as providing an ordinary "kitchen sponge" in a pair of dress shoes, that is, only minimal impact absorption is provided. Moreover, the foam cushion material does little to stabilize the foot within the shoe. In particular, the foam cushion has no stability along the edge of the midsole, an area which without support can cause the foot to roll over upon impact with the ground (pronation and supination). Such rollover is a major cause of injury during athletic activity.
On the other hand, a midsole that does not provide adequate cushion to the foot and does not absorb impact can itself cause injury to the foot. Therefore, a balance between comfort and stability is essential in any performance athletic shoe.
Various attempts have been made to "stabilize" foam midsoles. One example is shown in U.S. Pat. No. 4,506,462 to Cavanaugh which utilizes different density foam in select areas of the midsole. One problem associated with this type of stabilizer is that the harder density foam is often so hard that it does not compress at all under the forces encountered during typical athletic activity. Other attempts, such as U.S. Pat. No. 4,402,146 to Parracho et al. have provided a tab made of a material harder than the midsole adjacent the heel to prevent heel "rollover." A problem with this type of stabilizer is that the tab is often a heavy, solid plastic device that is so hard and unyielding that it can itself cause injury to the foot if the foot contacts the ground improperly, for example. Some, such as U.S. Pat. No. 4,297,796 to Stirtz et al. bond a mesh web to the exterior of a conventional cushion layer midsole to distribute shock along the sole of a shoe. Others, such as U.S. Pat. No. 4,535,553 to Derderian et al. and U.S. Pat. No. 4,774,774 to Allen, Jr. encase a shock absorbing insert within a conventional cushion midsole of a shoe. Thus, the need exists for a stabilizer for the midsole of a shoe which provides optimum peripheral rigidity while maintaining a central cushioning core.